You really should check for 3 conditions, no images, 1 image, more than 1 image
$images = get_field(gallery_field);
// the above will return something
// other than an array
// if there are no images
if (is_array($images)) {
// at lease 1 image
if (count($images) == 1) {
// only 1 image
} else {
// more than 1 image
}
}